Restore default tabbar style when custom stylesheet is disabled

Test Plan:

• Open new tab to show tabbar
• Enable Use user-defined stylesheet in Konsole Settings
• Set some stylesheet file; example contents:

QTabBar::tab { background-color: #0f0; border: 0; color: #000; }

• Apply settings
• Disable Use user-defined stylesheet in Konsole Settings
• Apply settings

Actual result:
Custom style is still used by tabbar
Expected result:
Default tabbar style is restored

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: hindenburg, konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D17605

(cherry picked from commit 5260daa66e4f5876b8e7b11d70581a0a6f46d180)